Composite Structure

ABSTRACT

A composite structure, such as a structural member ( 12 ), is formed by the steps of knitting a three dimensional pre-form ( 17 ) from one or more selected fibres such as natural fibres selected from hemp, cotton, flax, jute and synthetic fibres such as boron aramide fibres, carbon fibres, glass fibres and polymer based fibres, using a three dimensional knitting machine ( 16 ). The pre-form ( 17 ) has a shape commensurate with that of the structure ( 12 ) to be formed. The pre-form ( 17 ) is caused to take its three dimensional shape by inflating or otherwise expanding the pre-form such as by injecting a foamable synthetic plastics material into the interior of the pre-form, and thereafter fixing the shape of the shaped fibres to form the composite structure.

FIELD OF THE INVENTION

This invention relates to composite structures and relates particularly to structures that can be formed of a variety of materials but for which composites provide improved properties and cost advantages.

The present invention will be described with particular reference to its application to the manufacture of a composite structure that may be utilised in the construction of automobile fixtures or fittings, or fixtures or fittings for other vehicles. It will be appreciated, however, that the invention has a wider application, and any description of specific application should not be considered limiting in any form. Thus, the invention has wide application in construction, industry, building, aircraft construction and many other areas.

BACKGROUND OF THE INVENTION

Composites come in many forms and may broadly be described as the marriage or intermixing of two or more different materials in order to exploit and use the properties of each. These properties may include relatively low weight, temperature resistance, wear resistance, high strength and stiffness, including sheer, tension, compression and torsional strength, and many other properties that may be useful for a particular application.

Composite materials are formed from a variety of natural and synthetic fibres, such as fibreglass, Kevlar (trade mark), phenolic epoxy, carbon, arimid, hemp, cotton and many others, as well as resins, synthetic plastics materials of all types, rubber and many other construction materials.

Many composite materials are presently commercially available including commonly applied random fibre, reinforced, injection moulded plastics, and thermal compression moulded reinforced plastics. These commonly available materials, however, generally require expensive tooling.

In the automotive industry, for example, many automobile parts are formed of pressed and formed sheet steel structures. For example, many seat structures are formed of sheet steel pressed and/or formed to the required shape and form for structural rigidity and function. To be able to match the properties of such pressed and/or formed steel structures it is necessary to utilise advanced composites such as those employed in the aircraft, aerospace, military and motor sports industries.

While the composites industries have developed rapidly in those areas unhindered by cost competitiveness, a different situation applies in the broader manufacturing industries where a fundamental difference occurs between cost and performance. The automotive industries, however, whilst embracing composites in many areas, particularly in the high end of the market, mainstream automotive industries are substantially limited by long production cycle times, high labour and tooling costs of most high end composites as against the compromised strength and performance qualities of the lower end, less costly products.

High end advanced composites such as carbon-carbon composites, commonly used in elite motor sport and aerospace applications, require costly pre-processing of material laminates employing directional weaves and pre-impregnation of binder resins. These materials have a limited shelf life and thus impose strict cost quality management processes to ensure the final product consistency and integrity. The manufacture of composite structures using these high end materials requires labour intensive lay-up processes to achieve the desired directional orientation of the fibres and optimised strength in complex geometries. This is seen to be the most limiting and time consuming attribute of the conventional advanced composites manufacturing process. Another limiting aspect for structural parts is the requirement for costly mould tooling and large expensive autoclave ovens to cure the binder materials.

Many attempts have been made to reduce or eliminate the lay-up process of directionally controlled fibre reinforced composites. Most of these attempts have resulted in a compromise on the directionality of the fibres and hence a compromise in the performance of the product. Some of the more successful attempts have employed modular subassembly components that may be of uniform geometry and thus are more open to mass production methods. These subassembly components, however, still require extensive labour for final assembly and compromise the design flexibility with limitations on the final geometry.

Accordingly, there is a need to provide improved composite structures, and methods for their manufacture, which will facilitate the use of composites in a broad range of applications, particularly air craft manufacture, automobile manufacture and many other applications.

It is also desirable to provide composite structures that are consistently uniform.

It is also desirable to provide composite structures that require minimum labour in manufacture, assembly and production.

It is also desirable to provide composite structures which are relatively rigid and robust in use.

It is also desirable to provide composite structures which are relatively simple and economical to produce.

It is also desirable to provide composite structures that are able to be formed with the necessary structural strength, and with other properties required for their application.

It is also desirable to provide a method of forming composite structures which is able to be adapted to produce structures of a wide variety of shapes.

It is also desirable to provide methods of producing composite structures which utilise the various properties of fibres to be incorporated into the structure.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided a method of forming a composite structure comprising the steps of:

knitting a three dimensional pre-form using a three dimensional knitting machine, the pre-form having a shape commensurate with that of the structure to be formed, and the pre-form being knitted using one or more selected fibres,

causing the knitted pre-form to take the three dimensional shape,

fixing the shape, and

subsequently using the fixed shape to define the composite structure.

Preferably, the shape is fixed by impregnating or coating the fibres with a curable resin and subsequently curing the resin, or by curing or otherwise setting synthetic fibres to form the composite three dimensional pre-form.

According to another aspect of the invention there is provided a composite three dimensional pre-form formed by the method described above.

The invention also includes a knitted three dimensional pre-form formed by knitting composite fibres, which may be natural fibres or synthetic fibres into a knitted structure which, when inflated or otherwise caused to take its three dimensional shape, takes the shape of a composite structure to be formed.

Preferred embodiments of the invention utilise three dimensional knitting technology which enables pre-forms of complex shapes to be knitted from natural and/or synthetic fibres, such as those to which reference is made above.

The preferred embodiments are designed such that the knitted structure forms, in its three dimensional shape, a composite space frame having the shape of the composite structure to be formed. In the knitted product, the fibres are aligned and arranged to utilise the unique properties of the selected fibres used. Thus, fibres may be incorporated into the knitted product to provide strength in one or more directions and/or to provide stiffness in certain areas. Fibres may also be included in the knitted product which, when heated or subjected to microwaves or other forms of energy cure or set so as to retain the three dimensional shape of the knitted product.

In some preferred embodiments, the knitted product is caused to take its three dimensional shape by injecting a foam, such as a polyurethane, polypropylene, polystyrene, or any other suitable synthetic plastics foam material. The foam expands within the knitted product and causes the knitted fibres to take their knitted three dimensional shape.

In other embodiments, the pre-form is caused to take its three dimensional shape by inflating the pre-form using a gas, such as air, or another fluid.

In some preferred embodiments, the knitted product, expanded by foam injection or by inflation, is sprayed or coated with, or dipped or immersed in a resin binder, such as a polyester or vinyl ester resin or any other suitable binder material.

In another form of the invention, one or more of the fibres forming the three dimensional knitted product (pre-form) is formed of a material able to be used in the knitted product but curable by application of UV light, heat, microwaves or other energy, to cure or harden and bind with other fibres to form the three dimensional composite structure.

In preferred embodiments of the invention, fibre orientation within the pre-form is controlled by the design of the knit with particular attention to controlling the fibre gauge, direction, density and interface. In areas of high stress or high load bearing regions of the formed structure, the knitted pre-form will have a pattern and thickness, and be formed of fibres appropriate to resist the stresses and loads. The design of the knitted pre-form will be tailored to ensure accurate and repeatable control of the overall shape of the formed structure when the pre-form is inflated or injected with foam. Adjustment of the geometry of the pre-form will be possible during the injection or inflation stage by inflating the pre-form within flexible jigs or fixtures defining the external shape of the composite structure. If the pre-form is inflated by injection with foam, the outer shape of the composite structure to be formed is determined by the jigs or fixtures supporting the pre-form during curing of the foam.

In one preferred form of the invention, resin coated fibres are introduced during the knitting stage, the fibres being coalesced with the application of heat. In another preferred embodiment, a resin matrix binder material is sprayed on the inflated pre-form and allowed to cure or caused to be cured by application of UV light, heat or other energy.

In a further preferred embodiment, the shaped, inflated pre-form is dipped or submerged into a solution of binder resin. Curing of the binder resin may be done by applying UV light, heat or, in the case of a two part epoxy, by applying the reactive agent. Alternatively, the resin may be simply allowed to cool to a set point whereby a chemical reaction results in curing.

A particular advantage of the composite structure in accordance with embodiments of the invention is the design flexibility and the possible geometries which are achievable. In addition, the embodiments of the invention may be formed with a large number of different materials both in terms of the fibres used to form the knitted pre-form and in terms of the binder.

In order that the invention is more readily understood, embodiments thereof will now be described with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a computer designed knitted pre-form product design;

FIG. 2 schematically illustrates the formation of the knitted pre-form using a three dimensional knitting machine;

FIG. 3 schematically illustrates pre-form inflation;

FIG. 4 schematically illustrates binder application;

FIG. 5 schematically illustrates binder curing;

FIG. 6 is an enlarged view of part of a tubular structure formed in accordance with the invention;

FIG. 7 is a cross-sectional view of the structure of FIG. 6; and

FIG. 8 illustrates the manufacture of a complex shaped composite structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 5 of the drawings, an embodiment of the invention is illustrated in which a seat frame structure 12 is fabricated. For this purpose, a computer-aided design (CAD) system 14 is utilised to produce the necessary design parameters and software to be used in a three dimensional knitting machine 16. The design of the pre-form 17 to be knitted is optimised in the CAD design in terms of knit structure, fibre properties, stress and load bearing regions of the formed structure 12. The design also optimises fibre gauge, direction and density and other parameters that are required to ensure that the knitted pre-form 17, when produced, has the required three dimensional shape (when inflated) to form the seat frame structure 12.

Three dimensional knitting machines are known and are known to be able to be controlled by computer programs to produce a seamless, three dimensional knitted product of complex shape utilising fibres of choice. In the particular embodiment illustrated, the pre-form 17 is knitted using a selection of fibres selected from, for example, natural fibres such as hemp, cotton, flax, jute and synthetic fibres such as boron aramide fibres, carbon fibres, glass fibres, polymer based fibres and the like. By varying the fibres used, the formation of the knit, the knit density and fibre direction, a plurality of structural features are able to be incorporated into the knitted pre-form 17 which, in the end product, results in a seat frame structure 12 having the necessary strength, stress and load bearing capacities in the required areas thereof.

Although the knitted pre-form may be knitted in a seamless single piece, in different embodiments of the invention, a number of separate, knitted pre-forms may be assembled together, such as in a jig or fixture, to produce a composite structure.

The knitted pre-form 17, as shown in FIG. 2, is inflated or caused to take its three dimensional shape by inflating with a gas such as air, as shown in FIG. 3. In an alternate embodiment, the pre-form 17 is injected with a foamable synthetic plastics material which is expandable within the interior space of the pre-form 17 to cause the pre-form to take up its knitted three dimensional shape. In this embodiment, the foam interior of the pre-form remains to form part of the formed composite seat frame structure 12.

Where a gas such as air is used to inflate the pre-form 17, the finished seat frame structure 12 is hollow and is able to receive electrical cables or act as conduit in addition to its function as a seat frame.

The inflated, or injected, pre-form 17 is then subjected to an infusion of a binder material which coats and permeates throughout the fabric of the knitted pre-form to wet and coat all the fibres. The infusion may be carried out by spraying the inflated, or injected, knitted pre-form 17 with a resin matrix binder material, as schematically illustrated in FIG. 4. Alternatively, the pre-form 17 may be immersed within a bath of binder material, such as a resin matrix, so that the binder material impregnates throughout the knitted fabric of the pre-form 17. The immersion or impregnation may take place either before inflation or after the pre-form has been inflated to its three dimensional shape. Once impregnated, the coated and inflated pre-form 17 is cured, as shown in FIG. 5, by application of ultraviolet light, heat or in the case of a two part epoxy binder material, by applying the reactive agent. Alternatively, the resin binder material may simply be allowed to cool to a set point which allows a chemical reaction to take place resulting in curing of the binder.

In a further embodiment, some of the fibres used in the knitted pre-form 17 may include resin coated fibres which integrate with other fibres of the pre-form and are curable to form the composite seat frame structure 12.

The particular binders used for particular embodiments will depend on the nature of the fibre and fibre combinations within the knitted pre-form fabric. Both synthetic and naturally occurring binder materials may be used in embodiments of the invention, the particular selection of binder being appropriate for the purpose for which the structure is constructed.

Referring to FIG. 6, a composite structure may include one more hollow, tubular members having an external shape of a particular form. The tubular member 21 shown in FIG. 6 has an elliptical shape, as shown in FIG. 7. In order to produce this elliptical shape, the knitted pre-form 17 includes a plurality of tethers or shape defining fibres (threads) 22 which extend between opposed wall portions 23 of the tubular member 21 to hold the member 21 to the desired shape when inflated. Without the provision of such tethers 22, the tubular member 21 would take up a substantially circular cross sectional shape.

By using internal and external tethers, relatively complex shapes are able to be knitted by the three dimensional knitting machine which incorporates the tethers as part of the knitting process.

Referring to FIG. 8, a complex structure 26 is formed in accordance with an embodiment of the invention. The structure 26 incorporates a broad, three dimensional surface with constrained geometry using external tethers 27 as well as internal tethers 28 in the core of the section.

In the fabrication of the structure 26, a pre-form 29 is produced by the three dimensional knitting machine 16, the pre-form 29 having in its inflated form, front and rear surfaces 31 and 32 which are spaced apart and retained at the predetermined spacing by the internal tethers 28. The front and rear walls 31 and 32 are also curved, as shown, and are maintained in the curved shape by the external tethers 29. When the space between the front and rear walls 31 and 32 is injected with foam, and/or when the front and rear walls 31 and 32 are impregnated or sprayed with a resin matrix/binder material, which is subsequently cured, the external tethers 27 may be removed from the formed structure 26.

It will be appreciated that structures of complex shapes within the capacity of three dimensional knitting machines may be formed in accordance with the present invention thereby avoiding the need for complex moulds and time consuming, labour intensive fibre lay-ups and resin impregnation. The methods and structure of the embodiments illustrated facilitates repeatable, mass produced, composite structure production. Where a compressive resistance may be required across a formed section, the material used for inflating the pre-form fabric, such as a foamable synthetic plastics material, may be selected to provide compressive resistance in the formed structure. The particular methods of inflating or injecting the pre-form to enable it to take its shape, together with design innovation of the construction, direction and tension of the knit to constrain the pre-form in a desired geometry, and the methods of supporting the structure for curing without the need for tooling, enhance the utility of the invention and its embodiments.

Resin impregnation, infiltration and curing methods may be varied to reduce production cycle times further enhancing the use of the invention in mass production.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 

1. A method of forming a composite structure including the steps of: knitting a three dimensional pre-form using a three dimensional knitting machine, the pre-form having a shape commensurate with that of the structure to be formed, and the pre-form being knitted using at least one selected fibre, causing the knitted pre-form to take the three dimensional shape by one of: inflating and expanding the knitted pre-form; fixing the shape, and subsequently using the fixed shape to define the composite structure.
 2. A method of forming a composite structure as claimed in claim 1 including the steps of fixing the shape by impregnating the fibres with a curable resin and subsequently curing the resin.
 3. A method of forming a composite structure as claimed in claim 1 including the step of fixing the shape by setting the shaped fibres to form the composite three dimensional pre-form.
 4. A method of forming a composite structure as claimed in claim 1 including the step of designing the structure such that the knitted pre-form forms, in its three dimensional shape, a composite space frame having the shape of the composite structure to be formed.
 5. A method of forming a composite structure as claimed in claim 1 including the step of knitting the pre-form using a mixture of natural and synthetic fibres.
 6. A method of forming a composite structure as claimed in claim 1 wherein the fibres are selected from the group consisting of hemp, cotton, flax, jute and synthetic fibres, including boron aramide fibres, carbon fibres, glass fibres and polymer based fibres.
 7. A method of forming a composite structure as claimed in claim 1 including the step of aligning and arranging the fibres when knitting the pre-form to utilise unique properties of the selected fibres whereby fibres are incorporated into the knitted product to at least one of: provide strength in at least one direction and provide stiffness in predetermined areas of the pre-form.
 8. A method of forming a composite structure as claimed in claim 1 including the step of selecting fibres included in the knitted pre-form which, when one of: heated, subjected to microwaves, and subjected to other forms of energy, one of: cure and set so as to retain the three dimensional shape of the knitted pre-form.
 9. A method of forming a composite structure as claimed in claim 1 including the steps of: causing the knitted pre-form to take its three dimensional shape by injecting a foamable synthetic plastics material into an interior of the knitted pre-form and causing the foam to expand within the knitted pre-form to cause the knitted fibres to take their knitted three dimensional shape.
 10. A method of forming a composite structure as claimed in claim 9 wherein the foamed synthetic plastics material comprises a material selected from the group consisting of polyurethane, polypropylene, and polystyrene foam.
 11. A method of forming a composite structure as claimed in claim 9 including the step of spraying the knitted pre-form expanded by the foam with a resin binder.
 12. A method of forming a composite structure as claimed in claim 9 including the step of dipping the knitted pre-form expanded by the foam in a resin binder.
 13. A method of forming a composite structure as claimed in claim 11 wherein the resin binder is one of a polyester and vinyl ester resin.
 14. A method of forming a composite structure as claimed in claim 1 wherein at least one of the fibres forming the three dimensional knitted pre-form is formed of a material able to be used in the knitted product but curable by application of one of UV light, heat, microwaves and other energy, to harden and bind with other fibres to form the three dimensional composite structure.
 15. A method of forming a composite structure as claimed in claim 1 including the step of controlling fibre orientation within the knitted pre-form by design of the knit with particular attention to controlling fibre gauge, direction, density and interface.
 16. A method of forming a composite structure as claimed in claim 1 including the step of forming the knitted pre-form so that, in areas of high stress or high load bearing regions of the formed structure, the knitted pre-form will have a pattern and thickness, and be formed of fibres appropriate to resist the stresses and loads.
 17. A method of forming a composite structure as claimed in claim 1 including the step of adjusting the geometry of the knitted pre-form during one of an injection and inflation stage by inflating the pre-form within one of flexible jigs and fixtures defining an external shape of the composite structure.
 18. A method of forming a composite structure as claimed in claim 1 including the step of defining an outer shape of the composite structure to be formed by one of jigs and fixtures supporting the pre-form during curing of the foam when the pre-form is inflated by injection with foam.
 19. A method of forming a composite structure as claimed in claim 1 including the step of introducing resin coated fibres during the knitting stage, the fibres being coalesced with the application of heat.
 20. A method of forming a composite structure as claimed in claim 1 including the step of spraying a resin matrix binder material onto the inflated pre-form, and allowing the pre-form to be cured by application of one of UV light, heat and other energy.
 21. A method of forming a composite structure as claimed in claim 19 including the step of curing a binder resin by applying one of: UV light, heat and, in the case of a two part epoxy, a reactive agent.
 22. A method of forming a composite structure as claimed in claim 19 including the step of allowing the resin to cool to a set point whereby a chemical reaction results in curing.
 23. A method of forming a composite structure as claimed in claim 1 including the step of knitting the pre-form in a seamless single piece.
 24. A method of forming a composite structure as claimed in claim 1 including the steps of knitting a number of separate, knitted pre-form sections and subsequently assembling the sections together to produce a composite pre-form.
 25. A method of forming a composite structure as claimed in claim 1 including the step of inflating the pre-form using one of air and other fluid.
 26. A method of forming a composite structure as claimed in claim 1 including the step of including a plurality of one of: tethers, shape defining fibres and shape defining threads which extend between opposed wall portions of the pre-form, at least one of internally and externally, to hold the pre-form to a desired shape when inflated.
 27. A composite three dimensional pre-form formed by the method as claimed in claim
 1. 28. A knitted three dimensional pre-form formed by knitting one of composite natural fibres and synthetic fibres into a knitted structure which, when one of inflated and expanded to take a three dimensional shape, takes the shape of a composite structure to be formed, the pre-form fibres being adapted to be fixed to maintain the three dimensional shape.
 29. A composite structure comprising: a knitted, three dimensional pre-form having a three dimensional shape commensurate with that of the structure to be formed, the pre-form being knitted using at least one selected fibre, and the fibres being fixed when the pre-form is one of inflated and expanded to take the three dimensional shape whereby the fixed three dimensional shape defines the composite structure.
 30. A composite structure as claimed in claim 29 wherein the three dimensional shape is fixed by impregnating the fibres with a curable resin and subsequently curing the resin while the pre-form is in the inflated condition.
 31. A composite structure as claimed in claim 29 wherein the three dimensional shape is fixed by setting the shaped fibres to form the composite three dimensional structure.
 32. A composite structure as claimed in claim 29 wherein the structure is designed such that the knitted pre-form forms, in its three dimensional shape, a composite space frame having the shape of the composite structure to be formed.
 33. A composite structure as claimed in claim 29 wherein the pre-form is knitted using a mixture of natural and synthetic fibres.
 34. A composite structure as claimed in claim 29 wherein the fibres are selected from the group consisting of hemp, cotton, flax, jute and synthetic fibres including boron aramide fibres, carbon fibres, glass fibres and polymer based fibres.
 35. A composite structure as claimed in claim 29 wherein the fibres are aligned and arranged to utilise unique properties of the selected fibres whereby fibres are incorporated into the knitted product to at least one of: provide strength in at least one direction and provide stiffness in predetermined areas of the pre-form.
 36. A composite structure as claimed in claim 29 wherein the fibres, when one of: heated, subjected to microwaves, and subjected to other forms of energy, one of: cure and set so as to retain the three dimensional shape of the knitted pre-form.
 37. A composite structure as claimed in claim 29 wherein a foamable synthetic plastics material is injected into the interior of the knitted pre-form and caused to expand within the knitted pre-form to cause the knitted fibres to take their knitted three dimensional shape.
 38. A composite structure as claimed in claim 36 wherein the foamed synthetic plastics material comprises a material selected from the group consisting of polyurethane, polypropylene, and polystyrene foam.
 39. A composite structure as claimed in claim 36 wherein the knitted pre-form expanded by a foam is sprayed with a resin binder which is subsequently cured.
 40. A composite structure as claimed in claim 36 wherein the knitted pre-form, expanded by a foam, is one of dipped and immersed in a resin binder.
 41. A composite structure as claimed in claim 39 wherein the resin binder is one of a polyester and vinyl ester resin.
 42. A composite structure as claimed in claim 29 wherein at least one of the fibres forming the three dimensional knitted pre-form is formed of a material able to be used in the knitted product but curable by application of one of UV light, heat, microwaves and other energy, to harden and bind with other fibres to form the three dimensional composite structure.
 43. A composite structure as claimed in claim 29 wherein the fibre gauge, direction, density and interface of the fibres is controlled within the knitted pre-form.
 44. A composite structure as claimed in claim 29 wherein the knitted pre-form is knitted so that, in areas of high stress or high load bearing regions of the formed structure, the knitted pre-form will have a pattern and thickness, and be formed of fibres appropriate to resist the stresses and loads.
 45. A composite structure as claimed in claim 29 wherein the geometry of the knitted pre-form during one of an injection and inflation stage is adjusted by inflating the pre-form within one of flexible jigs and fixtures defining the external shape of the composite structure.
 46. A composite structure as claimed in claim 37 wherein the outer shape of the composite structure to be formed is defined by one of jigs and fixtures supporting the pre-form during curing of the foam when the pre-form is inflated by injection with foam.
 47. A composite structure as claimed in claim 29 wherein resin coated fibres are introduced to the pre-form during the knitting stage, the fibres being coalesced with the application of heat.
 48. A composite structure as claimed in claim 29 wherein a resin matrix binder material is sprayed onto the inflated pre-form, and the pre-form is allowed to be cured by application of one of UV light, heat and other energy.
 49. A composite structure as claimed in claim 47 wherein a two part epoxy binder resin is cured by applying a reactive agent.
 50. A composite structure as claimed in claim 29 wherein the knitted pre-form is a seamless single piece.
 51. A composite structure as claimed in claim 29 wherein the knitted pre-form is made in a number of separate, knitted pre-form sections and subsequently assembled to produce a composite pre-form.
 52. A composite structure as claimed in claim 29 wherein the pre-form is inflated using one of air and other fluid.
 53. A composite structure as claimed in claim 29 wherein a plurality of one of: tethers, shape defining fibres and shape defining threads extend between opposed wall portions of the pre-form, at least one of internally and externally, to hold the pre-form to a desired shape when inflated. 54-56. (canceled) 